This project will continue to evaluate immunocompetent cells involved in host resistance to infection by intracellular bacteria. Listeria monocytogenes infection in the mouse is used as the model system. We are conducting experiments to determine why injection of either Listeria fractions or thioglycollate medium prior to infection with live Listeria decreases the mouse's ability to resist infection. Experiments to confirm this finding in vitro and to evaluate the mechanism are in progress. Another approach is selective propagation of Listeria specific T cells, whose function will be evaluated. Studies on biologic effects with Listeria fractions include their ability to function as mitogens and adjuvants, as antigens, and as activators of macrophages. Activation of macrophages to kill tumor cells is the criterion by which we measure macrophage activation.